The disclosure relates to micro-imaging devices, particularly dual-band (two-color) infrared photodetectors and imagers.
A photodetector sensitive to infrared wavelengths of light is also known as an infrared (IR) detector. IR detectors are used in a wide variety of applications including thermal detection for surveillance, tracking, night vision, search and rescue, non-destructive testing and gas analysis. Typically, an IR detector is formed as a device consisting of an array, usually rectangular, of IR-sensing photodetectors disposed at the focal plane of an imaging lens. Such a detector is commonly referred to as a focal plane array (FPA).
IR covers a broad range of wavelengths, and therefore the IR band is further divided into sub-bands including: near IR (0.75 to 1.0 μm); short-wavelength IR (1.0 to 3.0 μm) (SWIR band); mid-wavelength IR (3 to 5 μm) (MWIR band); and long-wavelength IR (8 to 14 μm) (LWIR band). Many IR-absorbent materials are only sensitive to a certain range of wavelengths of IR radiation. Furthermore, IR radiation in the range of 5 to 8 μm is not transmitted well in the atmosphere. Therefore, many contemporary IR detectors are being designed to sense longer wavelengths falling within the 3 to 5 μm atmospheric window portion of the MWIR band, as well as wavelengths falling within the LWIR band.
IR photodetectors are often produced using InSb and HgCdTe materials fabricated as p-n junction diodes. However, these thermal detectors require cooling to cryogenic temperatures of around 77K, which is complex, energy and volume consuming, and costly. The cryogenic temperatures are primarily used to reduce the dark current generated in, for example, the p-n junctions of the bulk materials and at the surface of the material by Shockley Reed Hall (SRH) generation.
Dark current affects many photosensitive devices and is characterized by a relatively small electric current that flows through the device even when no photons are entering the device. Dark current is one of the main sources of noise in image detectors such as IR detectors, and has traditionally been mitigated by operating the detectors at temperatures significantly below ambient (room) temperature. Dark current occurs due to the random generation of electrons and holes within the device. In the depletion region of certain IR photodetectors, the activation energy for generation is lowest, limiting the suppression of noise with reduced temperature.
The dominant approach to two-color IR detectors is currently the mercury-cadmium-telluride (HgCdTe, or referred to as “MCT”). The MCT approach can be well suited to MWIR and LWIR detection because the band gap of such materials—which depends on the mixture of mercury and cadmium—can be tuned by more than an order of magnitude (from less than 0.1 eV to greater than 1.0 eV). This ability to adjust the band gap based on the makeup of a semiconductor alloy is known as “band gap engineering.”
Photodetectors fabricated from MCTs and other semiconductor materials can be produced using deposition techniques such as liquid phase epitaxy or molecular beam epitaxy (MBE). As an illustration of such a process, MBE can be used to grow an HgCdTe layer under high-vacuum conditions starting with a CdZnTe (CZT) substrate that has a lattice spacing which closely matches HgCdTe. Such matching of lattice spacing is known as lattice matching. The MBE system evaporates and deposits Hg, Cd and Te onto the substrate in such a fashion that the mixtures of Hg, Cd, Te, and any additional dopants, can be precisely controlled-thus enabling band gap engineering of the resulting semiconductor layer.
The development of multispectral detectors based on MCT has been hindered by the size and cost of commercially available substrates as well as the difficulties associated with uniformity and correctability of the passivated p-n junction devices.
One approach to addressing the issue of dark current involves using Ga-free strained-layer superlattice (SLS) materials in barrier IR detectors. The strain of SLS materials facilitates suppression of inter-band tunneling and Auger recombination processes that lead to dark current. Moreover, the larger effective mass in SLS systems can lead to a reduction of tunneling current compared with MCT detectors of the same band gap. The barrier detector structure provides a unipolar surface (either n-type or p-type) that can be passivated. Ga-free SLS systems (such as InAs/InAsSb) maintain the ability to reach the LWIR absorption bands on GaSb substrates without suffering the surface passivation issues that plagued earlier GaSb/InAs type-II superlattice absorbing materials. However, the nature of Ga-free SLS detectors results in relatively low absorption strength and hole mobility which can affect the ultimate quantum efficiency that is attainable as well as the modulation transfer function.